Improved chemical and genetic methods have made many enzymes, proteins, and other peptides and polypeptides available for use as drugs or biocatalysts having specific catalytic activity. However, limitations exist to use of these compounds.
For example, enzymes that exhibit specific biocatalytic activity sometimes are less useful than they otherwise might be because of problems of low stability and solubility in organic solvents. During in vivo use, many proteins are cleared from circulation too rapidly. Some proteins have less water solubility than is optimal for a therapeutic agent that circulates through the bloodstream. Some proteins give rise to immunological problems when used as therapeutic agents. Immunological problems have been reported from manufactured proteins even where the compound apparently has the same basic structure as the homologous natural product. Numerous impediments to the successful use of enzymes and proteins as drugs and biocatalysts have been encountered.
One approach to the problems that have arisen in the use of polypeptides as drugs or biocatalysts has been to link suitable hydrophilic or amphiphilic polymer derivatives to the polypeptide to create a polymer cloud surrounding the polypeptide. If the polymer derivative is soluble and stable in organic solvents, then enzyme conjugates with the polymer may acquire that solubility and stability. Biocatalysis can be extended to organic media with enzyme and polymer combinations that are soluble and stable in organic solvents.
For in vivo use, the polymer cloud can help to protect the compound from chemical attack, to limit adverse side effects of the compound when injected into the body, and to increase the size of the compound, potentially to render useful compounds that have some medicinal benefit, but otherwise are not useful or are even harmful to an organism. For example, the polymer cloud surrounding a protein can reduce the rate of renal excretion and immunological complications and can increase resistance of the protein to proteolytic breakdown into simpler, inactive substances.
However, despite the benefits of modifying polypeptides with polymer derivatives, additional problems have arisen. These problems typically arise in the linkage of the polymer to the polypeptide. The linkage may be difficult to form. Bifunctional or multifunctional polymer derivatives tend to cross-link proteins, which can result in a loss of solubility in water, making a polymer-modified protein unsuitable for circulating through the bloodstream.of a living organism. Other polymer derivatives form hydrolytically unstable linkages that are quickly destroyed on injection into the bloodstream. Some linking moieties are toxic. Some linkages reduce the activity of the protein or enzyme, thereby rendering the protein or enzyme less effective.
The structure of the protein or enzyme dictates the location of reactive sites that form the loci for linkage with polymers. Proteins are built of various sequences of alpha-amino acids, which have the general structure
The alpha amino moiety (H2N—) of one amino acid joins to the carboxyl moiety (—COOH) of an adjacent amino acid to form amide linkages, which can be represented as
where n can be hundreds or thousands. The terminal amino acid of a protein molecule contains a free alpha amino moiety that is reactive and to which a polymer can be attached. The fragment represented by R can contain reactive sites for protein biological activity and for attachment of polymer.
For example, in lysine, which is an amino acid forming part of the backbone of most proteins, a reactive amino (—NH2) moiety is present in the epsilon position as well as in the alpha position. The epsilon —NH2 is free for reaction under conditions of basic pH. Much of the art has been directed to developing polymer derivatives having active moieties for attachment to the epsilon —NH2 moiety of the lysine fraction of a protein. These polymer derivatives all have in common that the lysine amino acid fraction of the protein typically is modified by polymer attachment, which can be a drawback where lysine is important to protein activity.
Poly(ethylene glycol), which is commonly referred to simply as “PEG,” has been the nonpeptidic polymer most used so far for attachment to proteins. The PEG molecule typically is linear and can be represented structurally asHO—(CH2CH2O)nCH2CH2—OHor, more simply, as HO-PEG-OH. As shown, the PEG molecule is difunctional, and is sometimes referred to as “PEG diol.” The terminal portions of the PEG molecule are relatively nonreactive hydroxyl moieties, —OH, that can be activated, or converted to functional moieties, for attachment of the PEG to other compounds at reactive sites on the compound.
For example, the terminal moieties of PEG diol have been functionalized as active carbonate ester for selective reaction with amino moieties by substitution of the relatively nonreactive hydroxyl moieties, —OH, with succinimidyl active ester moieties from N-hydroxy succinimide. The succinimidyl ester moiety can be represented structurally as
Difunctional PEG, functionalized as the succinimidyl carbonate, has a structure that can be represented as

Difunctional succinimidyl carbonate PEG has been reacted with free lysine monomer to make high molecular weight polymers. Free lysine monomer, which is also known as alpha, epsilon diaminocaproic acid, has a structure with reactive alpha and epsilon amino moieties that can be represented as

These high molecular weight polymers from difunctional PEG and free lysine monomer have multiple, pendant reactive carboxyl groups extending as branches from the polymer backbone that can be represented structurally as

The pendant carboxyl groups typically have been used to couple nonprotein pharmaceutical agents to the polymer. Protein pharmaceutical agents would tend to be cross linked by the multifunctional polymer with loss of protein activity.
Multiarmed PEGs having a reactive terminal moiety on each branch have been prepared by the polymerization of ethylene oxide onto multiple hydroxyl groups of polyols including glycerol. Coupling of this type of multi-functional, branched PEG to a protein normally produces a cross-linked product with considerable loss of protein activity.
It is desirable for many applications to cap the PEG molecule on one end with an essentially nonreactive end moiety so that the PEG molecule is monofunctional. Monofunctional PEGs are usually preferred for protein modification to avoid cross linking and loss of activity. One hydroxyl moiety on the terminus of the PEG diol molecule typically is substituted with a nonreactive methyl end moiety, CH3—. The opposite terminus typically is converted to a reactive end moiety that can be activated for attachment at a reactive site on a surface or a molecule such as a protein.
PEG molecules having a methyl end moiety are sometimes referred to as monomethoxy-poly(ethylene glycol) and are sometimes referred to simply as “mPEG.” The mPEG polymer derivatives can be represented structurally asH3C—O—(CH2CH2O)n—CH2CH2-Zwhere n typically equals from about 45 to 115 and -Z is a functional moiety that is active for selective attachment to a reactive site on a molecule or surface or is a reactive moiety that can be converted to a functional moiety.
Typically, mPEG polymers are linear polymers of molecular weight in the range of from about 1,000 to 5,000. Higher molecular weights have also been examined, up to a molecular weight of about 25,000, but these mPEGs typically are not of high purity and have not normally been useful in PEG and protein chemistry. In particular, these high molecular weight mPEGs typically contain significant percentages of PEG diol.
Proteins and other molecules typically have a limited number and distinct type of reactive sites available for coupling, such as the epsilon —NH2 moiety of the lysine fraction of a protein. Some of these reactive sites may be responsible for a protein's biological activity. A PEG derivative that is attached to a sufficient number of such sites to impart the desired characteristics can adversely affect the activity of the protein, which offsets many of the advantages otherwise to be gained.
Attempts have been made to increase the polymer cloud volume surrounding a protein molecule without further deactivating the protein. Some PEG derivatives have been developed that have a single functional moiety located along the polymer backbone for attachment to another molecule or surface, rather than at the terminus of the polymer. Although these compounds can be considered linear, they are often referred to as “branched” and are distinguished from conventional, linear PEG derivatives since these molecules typically comprise a pair of mPEG- molecules that have been joined by their reactive end moieties to another moiety, which can be represented structurally as -T-, and that includes a reactive moiety, -Z, extending from the polymer backbone. These compounds have a general structure that can be represented as

These monofunctional mPEG polymer derivatives show a branched structure when linked to another compound. One such branched form of mPEG with a single active binding site, -Z, has been prepared by substitution of two of the chlorine atoms of trichloro-s-triazine with mPEG to make mPEG-disubstituted chlorotriazine. The third chloride is used to bind to protein. An mPEG disubstituted chlorotriazine and its synthesis are disclosed in Wada, H., Imamura, 1., Sako, M., Katagiri, S., Tarui, S., Nishimura, H., and Inada, Y. (1990) Antitumor enzymes: polyethylene glycol-modified asparaginase. Ann. N.Y. Acad. Sci. 613, 95-108. Synthesis of mPEG disubstituted chlorotriazine is represented structurally below.

However, mPEG-disubstituted chlorotriazine and the procedure used to prepare it present severe limitations because coupling to protein is highly nonselective. Several types of amino acids other than lysine are attacked and many proteins are inactivated. The intermediate is toxic. Also, the mPEG-disubstituted chlorotriazine molecule reacts with water, thus substantially precluding purification of the branched mPEG structure by commonly used chromatographic techniques in water.
A branched mPEG with a single activation site based on coupling of mPEG to a substituted benzene ring is disclosed in European Patent Application Publication No. 473 084 A2. However, this structure contains a benzene ring that could have toxic effects if the structure is destroyed in a living organism.
Another branched mPEG with a single activation site has been prepared through a complex synthesis in which an active succinate moiety is attached to the mPEG through a weak ester linkage that is susceptible to hydrolysis. An mPEG-OH is reacted with succinic anhydride to make the succinate. The reactive succinate is then activated as the succinimide. The synthesis, starting with the active succinimide, includes the following steps, represented structurally below.

The mPEG activated as the succinimide, mPEG succinimidyl succinate, is reacted in the first step as shown above with norleucine. The symbol -R in the synthesis represents the n-butyl moiety of norleucine. The mPEG and norleucine conjugate (A) is activated as the succinimide in the second step by reaction with N-hydroxy succinimide. As represented in the third step, the mPEG and norleucine conjugate activated as the succinimide (B) is coupled to the alpha and epsilon amino moieties of lysine to create an mPEG disubstituted lysine (C) having a reactive carboxyl moiety. In the fourth step, the mPEG disubstituted lysine is activated as the succinimide.
The ester linkage formed from the reaction of the mPEG-OH and succinic anhydride molecules is a weak linkage that is hydrolytically unstable. In vivo application is therefore limited. Also, purification of the branched mPEG is precluded by commonly used chromatographic techniques in water, which normally would destroy the molecule.
The molecule also has relatively large molecular fragments between the carboxyl group activated as the succinimide and the mPEG moieties due to the number of steps in the synthesis and to the number of compounds used to create the fragments. These molecular fragments are sometimes referred to as “linkers” or “spacer arms,” and have the potential to act as antigenic sites promoting the formation of antibodies upon injection and initiating an undesirable immunological response in a living organism.